1. Field of the Invention
The invention relates in general to transport refrigeration systems, and more specifically to transport refrigeration systems for straight trucks.
2. Description of the Prior Art
Refrigeration systems for medium-sized straight trucks include a single refrigeration circuit and first and second compressors which are selectively connectable to the single refrigeration circuit. The first compressor, which is located in the truck engine compartment, is driven by the truck engine while the truck is delivering a refrigerated load. The second compresser, which is located in a condenser section of the refrigeration unit, is driven by an electrical motor and a stand-by source of electrical potential when the truck is at a terminal and the cargo space requires refrigeration. Defrost of an evaporator section of the refrigeration unit is accomplished by diverting the hot gas discharge of the operative compressor from its normal path through the condenser section, directly into the evaporator section. When the refrigeration system is switched from a normal refrigerant path which is followed during a cooling cycle, to a "defrost path", the suction and discharge pressures are relatively low and they increase very slowly, which extends defrost time. On electrical stand-by in a truck terminal, it is common to aid hot gas defrost by energizing relatively large wattage resistors which are provided in heat exchange relation with an evaporator coil to be defrosted. The truck electrical system is not adequate for energizing these resistors when the truck is away from the terminal.
Refrigeration units for straight trucks do not have a dedicated internal combustion engine such as commonly provided on tractor-trailer refrigeration units. Thus, there is no hot engine coolant from a dedicated engine which may be used to enhance defrost while the truck is away from a terminal.